1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus which displays a stereoscopic image.
2. Related Art
A method of recording a stereoscopic image by using some method and reproducing it as a stereoscopic image is known. This method is called integral photography (hereafter referred to as IP method as well) or light ray reproduction method, and a large number of parallax images are displayed in this method. It is supposed that an object is viewed with left and right eyes. When a point A located at a short distance is viewed, an angle formed by the point A and the left and right eyes is denoted by α. When a point B located at a long distance is viewed, an angle formed by the point B and the left and right eyes is denoted by β. The angles α and β vary depending upon the position relation between the substance and the viewer. The difference (α−β) is called binocular parallax. Human being is sensitive to the binocular parallax and is able to conduct stereoscopic viewing.
In recent years, development of stereoscopic image display apparatuses without glasses has been promoted. Many of them are made possible by using the ordinary two-dimensional plane display device (for example, a liquid crystal display device), placing an optical plate formed of a lenticular lens or a slit on the front or back of the plane display device, utilizing the binocular parallax described above, and controlling angles of light rays from the plane display device so as to cause light rays to appear to be illuminated from objects located several cm before and behind the plane display device when viewed by a viewer. This is because it has become possible to obtain an image which is high in definition to some degree even if light rays of the plane display device are distributed to several kinds of angles (called parallaxes), owing to implementation of the two-dimensional display device having a higher definition. A three-dimensional (hereafter referred to as 3D as well) display method implemented by thus applying the IP method to the plane display device is called II (integral imaging) scheme. In the II scheme, the number of light rays illuminated from one lens corresponds to the number of element image groups. The number of the element image groups is typically called number of parallaxes. In each lens, parallax rays are illuminated in parallel.
In the II scheme, the viewer views different images: γ which is an image of 1 parallax, β which is an image of 2 parallaxes, and a which is an image of 3 parallaxes, according to the position of the viewer or the angle at which the viewer views. Therefore, the viewer perceives a solid body by parallax between the right eye and the left eye. If a lenticular lens is used as the optical plate, there is a merit that the display is bright because the utilization efficiency of light is high as compared with a slit.
In general, a liquid crystal display device is frequently used as a plane display device used in a stereoscopic image display apparatus. Fundamental properties concerning propagation of light in liquid crystal are described in, for example, “The foundation of liquid crystal and display application” written by Katsumi Yoshino and published by CORONA PUBLISHING CO., LTD., pp. 43-44. Hereafter, the fundamental properties will be described briefly. In the liquid crystal, each molecule takes a long and slender shape. Anisotropy of the refractive index occurs in a lengthwise direction of the molecule called director. For example, many of molecules in nematic liquid crystal are long and slender molecules. Their major axis directions are aligned and oriented. However, position relations of the molecules are random. Even if the orientation directions of molecules are in alignment, the absolute temperature is not zero degree, and consequently they are not perfectly parallel and there is fluctuation to some degree. Viewing a local region, it can be said that molecules are aligned in nearly one direction. When a region which is small enough macroscopically but large enough as compared with the size of the liquid crystal molecules is supposed, the average orientation direction of the molecules in that region is represented by using a unit vector, and it is referred to as director or orientation vector. An orientation in which the director becomes nearly parallel to the substrate is referred to as homogeneous orientation.
It is supposed that light is incident on optically uniaxial liquid crystal so as to have a propagation direction which forms an angle of θ with the optic axis. In a part which is outside of a medium and which has an isotropic refractive index of no, light propagates in a direction perpendicular to the wavefront. In the medium as well, an ordinary ray propagates in the normal line direction of the wavefront in the same way as the outside of the medium. As for an extraordinary ray, however, its energy propagation direction in the medium becomes a direction of φ with respect to the optic axis. In the medium plane, therefore, light propagates in a direction of θ-φ which is the direction of polarization. In other words, in the medium, the ordinary ray and the extraordinary ray which are orthogonal to each other in polarization direction propagate in different directions.
Furthermore, one of the greatest features of liquid crystal is optical anisotropy. Especially, since the degree of freedom in the molecule arrangement is high as compared with other anisotropic media such as crystal, the difference in refractive index between the major axis and the minor axis which is a criterion of double refraction is great.
As described in WO 2003/015424A1, there is a two-dimensional image/three-dimensional image changeover display device in which the effect of the lens is electrically lost by adding means which controls anisotropic lens and the polarization direction to a plane display device. A substance having the double refraction property is put into a lens, and an isotropic substance is put into an opposite position. As a result, light rays in a direction having a refractive index difference are converged by the lens, and light rays in a direction having no refractive index difference form a two-dimensional image. In the WO 2003/015424A1, however, a method for preventing three-dimensional image display from being degraded with respect to an angle at which the viewer views in a lens ridgeline direction is not described.
A stereoscopic image display apparatus disclosed in JP-A No. 2000-503424 (KOKAI) includes a display device (for example, a matrix type liquid crystal display panel) having a display face formed of pixels arranged in a matrix form, and lenticular means which is disposed on the output side of the display device, which has an array of lenticular elements passing through outputs of various pixels, which forms at least one stereoscopic view, and which causes the stereoscopic views to be visible to respective eyes of the viewer. In this stereoscopic image display device, the lenticular means contains an electrooptic material having an electrically variable refractive index, and a high resolution two-dimensional image can be displayed by selectively switching the refractive indexes so as to remove the action of the lenticular elements. In JP-A No. 2000-503424 (KOKAI), however, a method for preventing three-dimensional image display from being degraded with respect to an angle at which the viewer views in a lens ridgeline direction is not described.
In a stereoscopic image display apparatus obtained by utilizing one-dimensional integral imaging scheme in a stereoscopic image display apparatus, lights rays of a three-dimensional image are controlled only in the lateral direction of the viewer, i.e., ridgelines of a plurality of lens arrays are placed in the longitudinal direction and light rays are controlled only in the lateral parallax direction. It is now supposed that the viewer watches a stereoscopic image display apparatus in a standing posture when the stereoscopic image display apparatus is large and the view angle range is wide and consequently the depression angle changes according to the person's height. In other words, it is supposed that a plane image display apparatus is placed on a desk and the viewer views the display screen in a depression angle direction. Since the viewer views the stereoscopic image display apparatus not only from the front but also from obliquely above, a gap (distance) between a lens face used as an optical plate of the stereoscopic image display apparatus and a plane display device disposed at the back of the lens to display an elemental image becomes longer as compared with the display face is viewed from the front. Therefore, the gap becomes greater than the focal length of the lens, and the crosstalk increases, resulting in a problem that a viewing angle at which a favorable three-dimensional image can be viewed decreases.
On the other hand, if the gap between the lens and the plane display device located behind the lens in the stereoscopic image display apparatus is made nearly equal to the focal length, a favorable stereoscopic image in which adjacent elemental images are not mixed is obtained when the stereoscopic image display apparatus is viewed from the viewer's side.